You will not be added to the stand by list until you check in. You would not be a passenger on the flight prior to getting a confirmed seat - and thus not processed by EUA. EUA runs at 24 hours then 3 hours before flight time. Standing by after the 3 hour mark - just puts you on the standby list for processing by the gate agent. Others - Silvers, non-Elite Y etc would be likely processed at the 3 hour mark if wide open.

If you have a way to change sooner - then I would. Not sure how you can can change a day ahead for $65?